Remarkable, especially considering that crows, if not ravens, are highly susceptible to the West Nile virus. California Department of Public Health statistics show more dead crows than any other bird species testing positive for West Nile: 1,792 in 2008; 468 last year. (Raven mortality was minor.) The disease devastated crow populations in the East and Midwest, but California populations weren't dented.

Much of the crow and raven boom is urban. Birder Josiah Clark has seen flocks of 90 ravens in San Francisco. City crows are hard to miss in Berkeley and elsewhere in the East Bay; they're certainly, noisily, all over our neighborhood.

What brings them here? Kevin McGowan of the Cornell Lab of Ornithology notes that they don't get shot in cities; they benefit from both federal protected status and local firearms ordinances. That alone may encourage boldness. Also, he says, cities tend to be warmer than the countryside, and have large trees for night roosting. Urban crows are less likely to encounter their mortal enemy, the great horned owl, and city lights let crows spot owls before the owls spot them.

There's food, too: not so much the landfill smorgasbord (more the gulls' beat) as the fast-food parking lot buffet. "We eat so much out of doors now that these very intelligent birds can access all those food scraps we just drop or toss on the street," said Dan Murphy, compiler of the San Francisco Christmas Count. Some people feed them on purpose, too.

University of Washington biologist John Marzluff studies crow population dynamics in Seattle: "Young crows are moving to the cities to exploit their riches." He estimates that 70 percent of Seattle's annual increase in crow numbers is dispersal from the suburbs.

Clark says the most vulnerable species are open-cup nesters: "Cavity nesters like chickadees and bluebirds would be the most protected from corvids." McGowan, though, says studies show that removing crows doesn't improve the nesting success of potential prey species: Robin populations nationwide have kept pace with increasing crow populations.

Crows may be having an impact on birds of prey. "They harass the heck out of ravens and raptors," said Murphy. He describes an apparently fatal crow-fu attack on a Cooper's or sharp-shinned hawk: "My wife and I were watching a hawk fly down the hill toward Pine Lake Park when a flock of crows came after it. They swooped on it in dives like you'd see in a World War I movie. Eventually one crow got up behind it and dived down. It passed just behind the hawk and slapped it at the base of the skull. The hawk turned once in the air and took a straight dive into the trees down by the park. No way it pulled up. That bird was unconscious or dead in the air."

What are your options? In cities, you can't shoot them, and killing them requires a depredation permit. A former neighbor of ours broadcast hawk calls from a redwood where crows had roosted en masse. Marzluff says residents and developers "can limit crows simply by reducing lawn cover and increasing shrub and tree cover in yards, parks and neighborhoods."